In many wireless communication systems, multiple-input multiple-output (MIMO) communication is an advanced antenna technique that is used to improve spectral efficiency and increase system capacity. Cellular radio telephone systems, such as Evolved Universal Terrestrial Access (E-UTRA), or Long Term Evolution (LTE), systems, and UTRA systems, such as Wideband Code Division Multiple Access (WCDMA) and High-Speed Packet Access (HSPA) systems, that are compliant with specifications by the Third Generation Partnership Project (3GPP), and wireless local area network (WLAN) systems, such as Wi-Fi systems and other systems, that are compliant with IEEE 802.11 and 802.16, are examples of communication systems that use MIMO communication to varying extents.
MIMO communication generally entails multiple antennas at both the transmitter side and the receiver side of a communication. The antenna configuration in a MIMO communication system is typically represented with a notation (M×N), where M is the number of transmit antennas and N is the number of receive antennas. MIMO antenna configurations commonly considered today include (2×1), (1×2), (2×2), (4×2), (8×2) and (8×4). The (2×1) and (1×2) configurations are special cases that are sometimes called transmit diversity and receiver diversity, respectively, and that are of particular interest for cellular radio telephone systems, among others.
MIMO communication enables spatial processing of transmitted and received signals that in general improves spectral efficiency, extends cell coverage, enhances user data rate, mitigates multi-user interference, etc. Different MIMO configurations have different benefits. For instance, the receiver diversity (1×2) configuration can improve cell coverage. For another example, the (2×2) configuration can increase peak user bit rate, even doubling the bit rate. Such improved data rate depends on whether the two communication channels between the transmitter and receiver are sufficiently uncorrelated so that the rank of the 2×2 MIMO channel matrix is 2. The rank of a matrix is the number of independent rows or columns of the matrix. In general, the average two-link data rate will be less than twice the data rate achieved for a single link due to correlation between the links.
In cellular radio telephony, for example, MIMO techniques have been widely studied and applied for downlink communication, i.e., communication from base stations or equivalent network nodes to user equipments (UEs). For example, the (2×2) configuration will be used in WCDMA Release 7, and E-UTRA systems will support several MIMO configurations in the downlink, including single-user MIMO (SU-MIMO) and multiple-user MIMO (MU-MIMO).
MIMO techniques have typically been used only for downlink transmission because they increase the complexity of both the transmitter and the receiver compared to single-input single-output (SISO) communication. For example on the radio frequency (RF) side, a transmitter can need several RF power amplifiers (PAs) and several transmit antennas depending on the MIMO configuration, and a receiver can need several receive antennas and several chains of RF signal processing components depending on the MIMO configuration. Moreover, each MIMO configuration adds complexity in the base-band signal processing of the transmitter and receiver. Nevertheless, downlink MIMO with multiple PAs and antennas is considered feasible in a base station because the base station has fewer constraints on form factor and battery life.
The multiple transmit antennas in a MIMO configuration can be used in several different ways, such as antenna-switching and beam-forming. In general, antenna-switching leads to less improvement in communication performance than beam-forming does, but antenna-switching configurations can be easier to implement. If a transmitter, such as a UE, has some information about its uplink communication channel, the transmitter can use that information to steer its transmitted signal in the direction of the receiver by beam-forming with its multiple antennas. The channel information would be fed back to the transmitter by the receiver, and so such operation is a closed-loop multi-antenna technique. Open-loop multi-antenna techniques are based on the assumption that the transmitter, such as a UE, does not have information about the uplink channel, and so the transmitter cannot use such information for beam-forming.
FIG. 1 is a block diagram of a transmitter 100 that is configured for MIMO by antenna-switching. As depicted in FIG. 1, the transmitter 100 includes a suitable modulator 102 that up-converts or otherwise impresses an input signal onto a carrier signal appropriate to the communication system. The modulated carrier signal generated by the modulator 102 is provided to a PA 104 that increases the power of the modulated carrier signal. The amplified signal generated by the PA 104 is directed to one of two antennas 106, 108 by operation of a suitable switch 110 that is controlled by a signal generated by an antenna selector 112. As the antenna(s) in many communication devices are used for both transmitting and receiving, FIG. 1 shows duplexers 114, 116 that separate signals received by the antennas 106, 108 from the signal to be transmitted and direct those received signals to a receiver portion (not shown) of the communication device.
With the antenna-switching architecture depicted in FIG. 1, a transmitter such as a UE in a cellular telephone system has only a single PA but can achieve transmit diversity by switching the output of the PA 104 between the multiple antennas 106, 108. It can be shown that the fading uplink communication channels seen by the respective antennas are different, which is to say that the antennas are partially uncorrelated, and so by switching antennas, the transmitter can exploit the diversity of the channels.
FIG. 2 is a block diagram of a portion 200 of a transmitter that is configured for antenna beam-forming using two antennas. The modulator is omitted from FIG. 2 simply for clarity, and so FIG. 2 shows two modulated signals 1, 2 that are provided to a beam-forming processor 202, which combines the modulated signals 1, 2 according to a suitable beam-forming algorithm. Beam-formed signals generated by the processor 202 are provided to PAs 204, 206, and respective antennas 208, 210.
The beam-forming processor 202 applies a beam-forming vector or matrix W to the modulated signals 1, 2 to be transmitted before those signals are provided to the PAs 204, 206, which for example can be configured either as two full-power PAs or as one full-power PA and one half-power PA or as two half-power PAs. A full-power PA enables the transmitter to reach a nominal maximum transmit power, e.g., 23 dBm.
As depicted in FIG. 2, the beam-forming processor 202 implements the beam-forming vector or matrix as a multiplication-and-summing network, in which multiplicative weights w1, w2, . . . , w4 are applied to the modulated signals and the weighted modulated signals are summed to produce the beam-formed signals provided to the PAs. The weights w1, w2, . . . , w4 of the beam-forming matrix, which is to say the values of the elements of the vector W, can be either pre-defined and provided to the transmitter by the receiver through suitable signaling in closed-loop transmit diversity (CLTD), or optimized by the transmitter in open-loop transmit diversity (OLTD) by exploiting information already available in the UE, for example. As a result of the matrix W, the signal power transmitted by the antennas can be steered into a selected direction in order to maximize the probability of correct reception.
Recently, the 3GPP has started work on specifications that call for uplink transmit diversity (2×1) MIMO for Release-11 UTRA systems and on uplink MIMO for Release-11 E-UTRA systems. In a UTRA communication system for example, a UE can implement uplink transmit diversity (ULTD) with both antenna-switching and beam-forming. CLTD in such systems is “network-controlled”, which is to say that the network commands the UE to use a particular MIMO configuration by a transmitted precoding indicator (TPI) that is sent from a base station to the UE over a downlink fractional channel (F-TPICH).
CLTD theoretically provides a benefit in terms of throughput, while the benefit of OLTD depends more on the channel conditions and on the antenna selection algorithm used by the transmitter. To complicate things further, the benefit of OLTD may generally be less than the benefit of beam-forming, but under some conditions, it can be more beneficial to use antenna-switching.
3GPP has recently decided that under CLTD the base station or NodeB decides how the UE is configured, and so the base station indicates to the UE which of several predetermined beam-forming vectors W the UE should use in order to optimize its communication performance. The predetermined vectors are vectors of phases only, and so do not exploit the possible degrees of freedom in the amplitudes of the vector elements. Another possible problem with network-based control is that the base station can decide on a UE transmitter configuration that is not optimal from the UE's point of view.
FIG. 3 illustrates that a transmitter having a beam-forming architecture as in FIG. 2 can become inefficient from the point of view of the transmitter's current consumption. FIG. 3 depicts an example of an efficiency-vs-output power curve for a typical PA, showing a central linear region that is typically used for operation between minimum and maximum power levels. It will be noted that the relationship is non-linear, and at low power levels, the PA is less efficient. Thus, current consumed by the PA is not efficiently converted into output power. Different PAs can have different efficiency-vs-power curves, of course, but such curves are typically non-linear at low enough power levels. Due to the non-linear behavior of PAs at low power as depicted in FIG. 3, there is effectively a power floor below which ULTD or any multi-antenna technique using multiple PAs is no longer efficient.
In order to overcome this problem, existing solutions for CLTD could be based on having the UE signal the network about the inefficiency of the UE's PAs and ask the network to switch off the ULTD feature. A drawback of such solutions is that the network typically is not required to follow information from the UE, and so the network is not required to switch off ULTD even when a UE asks for it. Moreover, the UE-base station signaling wastes useful system resources, especially if the signaling needs to follow the UE's transmit power level and so maximize usage of ULTD.
Although a UE or other transmitter that implements the architecture of FIG. 1 may not suffer the drawbacks of increased current consumption in the architecture of FIG. 2, a transmitter as in FIG. 1 also does not support beam-forming, which can provide more improved performance. Another way that a UE can use to solve its current consumption problem is to switch off transmit diversity altogether, but doing so does not allow the UE to exploit any of the benefits of transmit diversity.